Boolean this made arch wtih a plane
You need to model the part you want removed as the &quot;tool&quot; for the boolean modifier.
In the image, if I Boolean Difference the arch from the plane, it creates a cutout of the arch in the plane as you can see on the left. If you only want the inside part of the arch to be removed from the plane you have to model that part as the tool for the boolean modifier, as on the right.